Field of invention
This invention relates to a light-emitting apparatus having a GaN semiconductor light-emitting device. More particularly, the present invention relates to an improvement on a light-emitting apparatus having a semiconductor light-emitting device integrated by flip chip bonding.
GaN semiconductors have been attracting attention as a material for light-emitting device emitting blue to green light. A light-emitting diode comprising such a GaN semiconductor is incroporated into a light-emitting apparatus as follows. Since a general light-emitting diode uses an insulating sapphire substrate, a pair of electrodes (i.e., a negative electrode and a positive electrode) are provided on the upper side of the semiconductor layer.
A first lead frame has a mount, onto which a substrate or a light-emitting diode is adhered with the semiconductor layer up so that the semicoductor layer faces the dominant light emitting direction of the light-emitting apparatus. The negative electrode and the postive electrode of the light-emitting diode are connected to the first and second lead frames, respectively. These members are encapsulated in a transparent sealing resin, such as an epoxy resin.
In the above-described light-emitting apparatus, blue to green light generated in the light-emitting layer in the semiconductor layer is transmitted through the semiconductor layer and emitted outside or reflected on the mount of the first lead frame, transmitted through the semiconductor layer again, and emitted outside. Accordingly, the direction perpendicular to the semiconductor layer is the dominant light-emitting direction. For more details, JP-A-7-23558 can be referred to.
However, conventional sealing resin, such as an epoxy resin, is liable to color change, and the color change is accelerated by beat. According to the inventor's study, a sealing resin is observed to undergo gradual color change to yellow to browm around a semiconductor layer that is accompanied by heat generation while emitting light. The resin thus yellowed or browned absorbs blue to green light emitted from the light-emitting diode. That is, the brightness of a light-emitting apparatus (the quantity of light emitted outside through the sealing resin) is reduced with the progress of color change of the sealing resin.
In order to solve this problem, flip chip bonding has been proposed, in which a light-emitting diode is reversed, and the negative and the positive elcetrodes are mounted directly on the lead frames by soldering, etc. For more detail, JP-A-56-79482 can be referred to. According to this structure, the heat generated from the light-emitting device is easily dissipated outside through the lead frames so that the color change of the sealing resin can be suppressed.
The distance between the negative and the positive electrodes is governed by the size of the light-emitting device, which is as small as about 350 .mu.m. On the other hand, since a lead frame is formed by metal sheet working, the working precision is limited to the metal sheet thickness (e.g., 0.5 mn). Therefore, it is difficult to work a metal sheet in conformity to the distance between the electrodes of a light-emitting device. Assuming precise metal sheet working is possible, it is extremely difficult and impractical to fix the electrodes on the mounts of lead frames with positioning accuracy on the order of microns.
A light-emitting apparatus using a sealing resin having dispersed therein a fluoresent material to have the color of emitted light changed is known. For example, part of blue light generated from a light-emitting device is absorbed by the fluorescent material so that orange light is emitted from the fluorescent material. The blue light that does not pass through the fluorescent material and the orange light emitted from the fluorescent material are mixed up so that the Light-emitting device emits white light as a whole. However, it has been difficult with conventional Light-emitting apparatus of this type to secure color unchangeabililty of the emitted light for a long period of time because the sealing resin undergoes color change with time due to the heat generated in the Light-emitting device.